1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent device and a method of fabricating the same.
2. Discussion of the Related Art
In general, an organic electroluminescent device (ELD) emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. In comparison to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between the two states causes light to be emitted. Accordingly, the size and weight of the organic ELD are less than an LCD device. The organic ELD has other excellent characteristics, such as low power consumption, superior brightness, and a fast response time. Because of these characteristics, the organic ELD is seen as the display for the next-generation of consumer electronic applications, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is done with fewer processing steps, the organic ELD is much cheaper to produce than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively large amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by the width and thickness of conductive lines used in the structure. Further, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image on a large display with a relatively low power.
FIG. 1 is a cross-sectional view of an organic ELD in which emission light is transmitted toward a bottom direction according to a first comparative example of a related art. As shown in FIG. 1, an array element layer 14 including a thin film transistor (TFT) T is formed on a first substrate 12 including a pixel region P. A first electrode 16, an organic electroluminescent (EL) layer 18, and a second electrode 20 are formed over the array element layer 14. The organic EL layer 18 can separately display red, green, and blue colors for each pixel region P. A second substrate 28 faces the first substrate 12 and is spaced apart from the first substrate 12.
The first and the second substrates 12 and 28 are attached to each other with a sealant 26. The second substrate 28 includes a desiccant 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. The desiccant 22 is fixed by a holding element 25.
The first electrode 16 or lower electrode is transparent and the second electrode 20 or upper electrode is nontransparent. Light emission from the organic EL layer 18 is transmitted toward the first electrode 16. Therefore, this organic ELD may be referred as a bottom emission type.
A pad 40 is formed on the first substrate 12 in a periphery region of the seal pattern 26. An external circuit means EC is connected to the pad 40 to supply the array element layer 14 with signals. The external circuit means EC has a tape carrier package (TCP) 44 including a drive integrated circuit (IC) 42 directly connected to the pad 40 and a printed circuit board (PCB) 46 connected to the TCP 44 to supply the signals.
In the bottom emission type, the external circuit means EC is located adjacent to a top surface of the second substrate 28. More specifically, the PCB 46 is attached on a backside of the second substrate 28 and the TCP 44 connected to the pad 40 and the PCB 46 in a bent shape. However, it is substantially difficult to obtain high brightness in a transmissive region in this bottom emission type because the emission light is transmitted through the array element layer 14 having a plurality of opaque metal lines. Therefore, a top emission type organic ELD has been suggested in which light emission is transmitted in a top direction, which is in an opposite direction of the bottom emission type, in order to solve the above-mentioned problems.
FIG. 2 is a cross-sectional view of an organic ELD according to a second comparative example of the related art. As shown in FIG. 2, a top emission type organic ELD 80 includes an array element layer 64 on a substrate 62 having a pad 90, a first opaque electrode 66, an organic EL layer 68 on the first opaque electrode 66, and a second transparent electrode 70. For example, when the first electrode 66 acts as an anode and the second electrode 70 acts as a cathode, the first electrode 66 includes a transparent material having a high work-function and the second electrode 70 includes an opaque material having a low work-function. In this case, it is difficult for the first electrode 66 to be completely opaque and for the second electrode to be completely transparent. Therefore, the first electrode 66 also includes an opaque conductive material while the second electrode 70 is a metallic material thin enough to allow light transmission to occur.
A passivation layer 96 is formed on an entire surface of the second electrode 70 to protect the organic ELD 80. In other words, the passivation layer 96 covers the surfaces of the array element layer 64 and the organic EL layer 68. Moreover, as explained in reference to FIG. 1, an external circuit means 94 is connected to the array element layer 64 through a pad 90 in a periphery region of the substrate 62. In comparison with the bottom emission type organic ELD shown in FIG. 1, the external circuit means EC is located adjacent to a bottom surface of the substrate 62, as shown in FIG. 2. Therefore, the PCB 96 is mounted on a backside of the substrate 62 and the TCP 94 is located between the pad 90 of the substrate 62 and the PCB 96 as a bent shape corresponding to a side portion of the substrate 62.
The portion of the substrate 62 for attaching the TCP 94 having a drive IC 92 needs to be at least about 2˜3 mm. Moreover, it is difficult for the PCB 96 to be completely attached to the substrate 62. More specifically, a detachment phenomenon of the TCP 94 occurs as a result of impact tests for reliability. Further, the organic ELD 80 may easily overheat because the above-mentioned top emission type organic ELD has a structure in which heat generated by the organic EL layer 68 can not be readily dissipated.